Biomimetic pathways for assembling inorganic thin films and oriented mesoscopic silicate patterns through guided growth

ABSTRACT

A process directed to preparing surfactant-polycrystalline inorganic nanostructured materials having designed microscopic patterns. The process includes forming a polycrystalline inorganic substrate having a flat surface and placing in contact with the flat surface of the substrate a surface having a predetermined microscopic pattern. An acidified aqueous reacting solution is then placed in contact with an edge of the surface having the predetermined microscopic pattern. The solution wicks into the microscopic pattern by capillary action. The reacting solution has an effective amount of a silica source and an effective amount of a surfactant to produce a mesoscopic silica film upon contact of the reacting solution with the flat surface of the polycrystalline inorganic substrate and absorption of the surfactant into the surface. Subsequently an electric field is applied tangentially directed to the surface within the microscopic pattern. The electric field is sufficient to cause electro-osmotic fluid motion and enhanced rates of fossilization by localized Joule heating.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention generally relates to a process for preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns using a polycrystalline inorganicsubstrate. More specifically, this invention relates to biomimeticallyassembling inorganic thin films, and to the synthesis of mesostructuredfilm using a supramolecular assembly of surfactant molecules atinterfaces to template the condensation of an inorganic silica lattice.Additionally, this invention relates to forming an ordered silicatestructure within a highly confined space.

[0003] 2. Related Art

[0004] Biologically produced inorganic-organic composites such as bone,teeth, diatoms, and sea shells are fabricated through highly coupled(and often concurrent) synthesis and assembly. These structures areformed through template-assisted self-assembly, in which self-assembledorganic material (such as proteins, or lipids, or both) form thestructural scaffolding for the deposition of inorganic mateial. They arehierarchically structured composites in which soft organic materials areorganized on length scales of 1 to 100 nm and used as frameworks forspecifically oriented and shaped inorganic crystals (that is, ceramicssuch as hydroxyapatite, CaCo₃, SiO₂, and Fe₃O₄). In some cases,structurally organized organic surfaces catalytically or epitaxiallyinduce growth of specifically oriented inorganic thin films.

[0005] Most importantly, however, nature's way of mineralization usesenvironmentally balanced aqueous solution chemistries at temperaturesbelow 100° C. This approach provides an attractive alternative to theprocessing of inorganic thin films, especially in applications wheresubstrates cannot be exposed to high temperatures, or more generally inthe pursuit of increased energy efficiency.

[0006] Potential applications for dense, polycrystalline inorganic filmsspan a broad range of industries. These include the possibility ofapplying hard optical coatings to plastics in order to replace glass,abrasion-resistant coatings for plastic and metal components subject towear, and the deposition of oriented films of iron oxide phases for useas magnetic storage media. For many of these applications, conventionalceramic processing methods, which require high temperature sintering,cannot be used because of problems with substrate degradation.

[0007] A classic and a widely studied example of a biocomposite is thenacre of abalone shell, in which thin films of organic (<10 nm) andinorganic (<0.5 μm) phases are coupled together to produce a laminatedstructure with improved mechanical properties. Scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM) images ofthis material are shown in FIG. 1 of this application. Because of thisspecial architecture, composites such as nacre are simultaneously hard,strong, and tough. The core of the organic template is composed of alayer of β-chitin layered between “silk-like” glycine-and alanine-richproteins. The outer surfaces of the template are coated with hydrophilicacidic macromolecules rich in aspartic and glutamic acids. Recentstudies suggest that these acidic macromolecules alone are responsiblefor control of the polymorphic form and the morphology of the CaCO₃(calcite versus aragonite) crystals, although the role of the β-chitinsupported matrix on the lamellar morphology of the CaCO₃ layers overmacroscopic dimensions still remains to be determined.

[0008] Morphological and crystallographic analyses of the aragoniticthin layers of nacre by electron microdiffraction show thatc-axis-oriented aragonite platelets form a hierarchical tiling of atwin-related dense film with twin domains extending over three lengthscales. Superposition of the aragonite lattices on all three possiblesets of twins generates a new superlattice structure, which suggeststhat the organic template adopts a single-crystalline psuedobexagonalstructure. Although cellular activities leading to the self-assembly orthe organic template remain to be understood, the presence of organizedorganic template is essential to the assembly of the inorganic layer.

[0009] In recent years, a number of researchers have demonstrated theviability of this approach for the preferential growth of inorganiccrystals at the solid/liquid and liquid/air interfaces. Furthermore,through chemical modification of these interfaces, by adsorbingsurfactants or other reactive moieties, the crystal phase, morphology,growth habit, and even chirality of heterogeneously deposited inorganicscan be controlled.

[0010] Mann et al., Nature 332, 119 (1988), describes phase-specific,oriented calcite crystals grown underneath a compressed surfactantmonolayer at the air/water interface. Changing surfactant type or degreeof monolayer compression results in different crystal phases andorientations.

[0011] Pacific Northwest National Laboratories (PNNL), B. C. Bunker etal., Science 264, 48 (1994), describes chemically modifying solid metal,plastic, and oxide surfaces, and the selection of phase and orientationof the depositing crystalline inorganic at a variety of solid/liquidinterfaces. Bunker et al. describes the use of a self-assembledmonolayer (SAM) approach to coat metal and oxide substrates withsurfactant monolayers of tailored hydrophilicity. This is accomplishedby pretreating the substrates with a solution of functionalizedsurfactants, such as sulfonic acid-terminated octadecyl tricholorsilane,before precipitation of the inorganic phase. The choice of theterminating moiety on the surfactant tail determines surface charge andrelative hydrophobicity of the chemisorbed surfactant monolayer. In thisway, oxide and metal substrates can be modified to have the requiredsurface properties to promote inorganic film growth.

[0012] A. Kumar and G. M. Whitesides, Appl. Phys. Lett. 63, 2002 (1993);and A. Kumar, H. A. Biebuyck, G. M. Whitesides, Langmuir 10, 1498(1994), describe a microcontact printing method by which complex,designed SAM patterns may be transferred onto substrates with anelastomeric stamp. This approach sets up lateral variations in theγ_(is)-γ_(sl) value along the substrate and may be used to selectivelynucleate and grow inorganic phase on the finctionalized regions.

[0013] B. J. Tarasevich, P. C. Rieke, J. Lin, Chem. Mater. 8, 292(1996); and P. C. Rieke et al., Langmuir 10,619 (1994), describe thespatially resolved deposition of FeOOH mineral through an analogous SAMapproach by using electron and ion beam lithography to pattern the SAMlayer. This technique allows micrometer-scaled patterning of inorganicmaterials on a variety of substrates through confined nucleation andgrowth of inorganic films.

[0014] M. R. De Guire et al., SPIE Proc. in press; and R. J. Collins, H.Shin, M. R. De Guire, C. N. Sukenik, A. H. Heuer (unpublished) describethe use of photolithography to pattern the SAM layer prior toarea-selective mineralization of TiO₂, ZrO₂, SiO₂, or Y₂O₃ films.

[0015] Kim et al., Nature 376,581 (1995) describes an alternative to theSAM approach of micromolding in capillaries (MIMIC). In this process,submicrometer-scale patterning of inorganic films is achieved by placingan elastomeric stamp, containing relief features on its surface, intocontact with a substrate. Contact between the elastomeric stamp and thesubstrate forms a network of interconnected channels that may be filledwith an inorganic precursor fluid [such as poly(ethoxymethylsiloxane)]through capillary action. After the material in the fluid iscross-linked, crystallized, or deposited onto the substrate, theelastomeric stamp is removed to leave behind a patterned inorganic filmwith micro-structures complementary to those present in the mold.

[0016] S. Manne et al. Langmiur 10, 4409 (1994) and H. Gaub, Science270, 1480 (1995), have shown that three-dimensional surfactantstructures such as cylindrical tubules and spheres can be formed atsolid/liquid interfaces. Adsorbed hemi-micellar arrangements wereobserved on poorly orienting amorphous substrates, such as silica, andaligned tubular structures were observed on more strongly orientingcrystalline substrates such as mica and graphite. The latter substratesorient adsorbed surfactants through anisotropic attraction (either vander Waals or electrostatic) between the crystalline substrate and thesurfactant molecule. The amorphous silica substrate has no preferentialorientation for surfactant adsorption.

[0017] Aksay et al., Science 273, 892 (1996) describes a method for theformation of continuous mesoscopic silicate films at the interfacebetween liquids and various substrates. The technique used thesupramolecular assembly of surfactant molecules at interfaces totemplate the condensation of an inorganic silica lattice. In thismanner, continuous mesostructured silica films can be grown on manysubstrates, with the corresponding porous nanostructure determined bythe specifics of the substrate surfactant interaction. XRD analysisrevealed epitaxial alignment of the adsorbed surfactant layer withcrystalline mica and graphite substrates, and significant strain in themesophases silica overlayer. As the films grew thicker, accumulatedstrain was released resulting in the growth of hierarchical structuresfrom the ordered film. This method was used to form “nanotubules” withdimensions of ˜3 nm. Polymerization of the inorganic matrix around thesetubules leads to a hexagonally packed array of surfactant channels.

[0018] The aforedescribed techniques represent advances in the selectivenucleation growth of inorganic crystals with specific phase,orientation, and micropatterns. A significant advantage of thebiomimetic processing methods described above is the relatively lowprocessing temperatures involved (typically <100° C.) and the use ofwater rather than organic solvents. Both of these factors render suchmethods relatively environmentally benign. Although continuous films ofthese silicate materials can be formed, the orientation of the tubulesdepends primarily on the nature of the substrate-surfactant interactionand is difficult to control. Once films grow away from the orderinginfluence of the interface, chaotic, hierarchical structures arise.Additionally, there is no facility for organic material to adsorb ontoor to become incorporated within the growing inorganic structure or todo both.

[0019] There is thus a need for the development of low-cost lithographictechniques having the ability to pattern “designed” structural featureson the nanometer size scale. Such techniques are important in themanufacture of electronic, opto-electronic and magnetic devices withnanometer scaled dimensions. Technologies involving scanning electronbeam, x-ray lithography and scanning proximal probe are currently underdevelopment, but the practicality of these techniques remains uncertain.Although these continuous films hold much promise for a multitude oftechnological applications (e.g., oriented nanowires, sensor/actuatorarrays, and optoelectronic devices), a method of orienting thenanotubules into designed arrangements is clearly required for thisapproach to become viable as a nanolithographic tool. What is desiredand has not yet been developed is a method that allows the direction ofgrowth of these tubules to be guided to form highly aligned, designednanostructures. It would be desirable that the method is independent ofthe substrate-surfactant interaction and thus allows oriented structuresto be formed on any (non-conducting) substrate.

OBJECTS AND SUMMARY OF THE INVENTION

[0020] It is an object of this invention to provide a practical,low-cost lithographic process that has the ability to pattern “designed”structural features on the nanometer size scale.

[0021] It is an additional object of this invention to provide a processthat is useful in the manufacture of electronic, opto-electronic andmagnetic devices with nanometer scaled dimensions.

[0022] It is a further object of this invention to provide ananolithographic process that orients the nanotubules into designedarrangements.

[0023] It is still another object of this invention to provide ananolithographic process that allows the direction of growth of thesetubules to be guided to form highly aligned, designed nanostructures.

[0024] It is still another object of this invention to provide ananolithographic process that is independent of the substrate-surfactantinteraction and thus allows oriented structures to be formed on any(non-conducting) substrate.

[0025] All of the foregoing objects are achieved by the process of thisinvention. The process is directed to preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns. The process comprises:

[0026] a) forming a polycrystalline inorganic substrate having a flatsurface;

[0027] b) placing in contact with the flat surface of the substrate asurface having a predetermined microscopic pattern;

[0028] c) placing in contact with an edge of the surface having thepredetermined microscopic pattern, an acidified aqueous reactingsolution, the solution wicking into the microscopic pattern by capillaryaction, wherein the reacting solution comprises an effective amount of asilica source and an effective amount of a surfactant to produce amesoscopic silica film upon contact of the reacting solution with theflat surface of the polycrystalline inorganic substrate and absorptionof the surfactant into the surface; and

[0029] d) applying an electric field tangentially directed to thesurface within the microscopic pattern, the electric field beingsufficient to cause electro-osmotic fluid motion and enhanced rates offossilization by localized Joule heating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other important objects and features of the invention will beapparent from the following Detailed Description of the Invention takenin connection with that accompanying drawings in which:

[0031] FIGS. 1-7 are taken from Aksay et al., Science 273, 892 (1996)which describes the production of mesoscopic films without the requiredelectric field of this invention. These figures are incorporated in thisapplication for background and comparison.

[0032]FIG. 1(A) A scanning electron microscopy (SEM) (Phillips XL30FEG)image of fracture surface of aragonitic portion of abalone nacre showingaragonite (CaCO₃) platelets of ˜0.5 μm thick.

[0033]FIG. 1(B) A (TEM) Transmission electron microscopy (PhillipsCM200) image of the nacre cross section revealing a <10 nm thin organicfilm (marked “O”) between the aragonite platelets with their c-axisnormal to the organic template.

[0034]FIG. 2 shows SEM images of mesoscopic silica films grown at (A)mica/water, (B) graphite/water, and (C) silica/water interface for 24hours, respectively. Oriented tapes are observed on mica and graphite.The films grown at the silica/water interface are uniform initially(dark background) but spiral-like structures (light features) formlater.

[0035]FIG. 3 shows in situ AFM images of mesostructured films growing onmica, graphite, and amorphous silica substrates, respectively. AFMimages of the mica, graphite, and silica substrates used to growmesoscopic silica films are shown in the insets. (A) and (B) illustratethe periodic mica and graphite atomic lattices, respectively, onto whichCTAC adsorbs and orients; (C) reveals a smooth, amorphous silicasubstrate. Images of the films were obtained in “noncontact” mode,utilizing the electrical double layer force. (A) Meandering surfactanttubules on the mica substrate, 6.2 to 6.8 nm spacing, oriented parallelto the solid/liquid interface. Tubules are initially aligned along oneof the three next-nearest-neighbor directions of the mica oxygen latticedisplayed in inset. In the early stages of the reaction (<7 hours), thisorientation is preserved as tubtiles continue to assemble and grow awayfrom the interface coupled with silica polymerization. (B) On graphite,tubules align parallel to the substrate along one of three symmetry axesof the hexagonal carbon lattice shown in the inset. Unlike thestructures on mica, these do not meander but form rigid parallelstripes. (C) On amorphous silica, periodic dimples are observed ratherthan stripes, suggesting an orientation of the tubules away from theinterface.

[0036]FIG. 4 shows TEM images of a mesostructured silica film grown onmica. Both images are in a transverse orientation with respect to thefilm and reveal hexagonal packing of tubules aligned parallel to thesubstrate. The image in (A) reveals a slight elliptical distortion ofthe tubules suggesting that the films are strained, that is, compressedin the direction normal to the template.

[0037]FIG. 5(A) is a schematic illustration of the sequential mechanismof templated, supramolecular surfactant self-assembly on the micasurface (left), followed by intercalation and polymerization ofinorganic monomer to form a mesostructured composite (right). Assemblyof the first surfactant layer forms a template that defines thestructure of the subsequent film. On mica, electrostatic interactionsbetween the substrate and surfactant lead to complete cylinders thatmeander across the surface with a loose registry to the underlyingsubstrate lattice.

[0038]FIG. 5(B) is a schematic of a mesostructured silica on graphite.The rigid half-cylinder geometry on graphite occurs because attractive(hydrophobic and van der Waals) interactions between the graphitesurface and the surfactant tails cause them to adsorb horizontally.

[0039]FIG. 6(A) shows a SEM image of a hierarchically structuredmesoscopic silica film grown on a silica substrate. Although all of thefilms appear uniform at early stages of the reaction, once filmthicknesses exceed ˜0.5 μm, the ordering influence of the substratebecomes no longer important. Release of accumulated stain energy withinthe film leads to hierarchical structures, with tubule bundles wrappingaround each other in three dimensions on several length scales.

[0040]FIG. 6(B) shows a TEM image of a planar cross section of a filmgrown on silica. The cross section was taken through a macroscopic swirlin the film shown in (A) and reveals a spiraling and twistingarrangement of surfactant tubules.

[0041]FIG. 7 shows a grazing angle of incidence XRD data formesostructure silica film growing at the mica/aqueous solution phaseinterface (after 15 hours of reaction time) showing radial scans of twoBragg peaks, the (002) (filled circles) and the (101) (open squares,expanded by a factor of 1350). Growth of the surfactant film on afreshly cleaved mica substrate results in a highly aligned crystallinelattice, in which the (002) Bragg peak is oriented along the substratesurface normal, with a mosaic width that is less than 0.06°.Furthermore, the (101) Bragg peak is also azimuthally aligned within thesurface plane, such that the tubules are oriented along thenext-nearest-neighbor direction of the surface oxygen lattice, andhaving an in-plane mosaic width of ˜10°. Both of these observationsclearly suggest that the substrate has a strong orienting effect on theco-assembled film. Further evidence of the interaction between thesubstrate and the co-assembled film can be found in the exact Bragg peakpositions. Although bulk mesoscopic silica exhibits a hexagonal lattice,in which case Q₀₀₂=Q₁₀₁, for films grown on mica the radial peakposition of the (002) and (101) Bragg peaks are not equal (Q₀₀₂=0.139Å⁻¹ and Q₁₀₁=0.143 Å⁻¹), which implies that the film is strained. Fromthese data, we derive nearest-neighbor spacings of 52 and 50 Å. Thisstrain results in a ratio of lattice spacings of b/a=(b/{squareroot}3a)−1=3.7%, and an area per tube in the composite film of 2288 Å²(the inset defines the parameters a and b, and the lines show the modelfit). Strain in these films results from epitaxial mismatch between thefirst adsorbed surfactant layer and the periodic atomic lattice of thesubstrate.

[0042]FIG. 8 shows a scanning electron microscope (SEM) image of an“unconfined” mesoscopic silica film grown on an amorphous silicasubstrate. Once the film becomes thicker than ˜0.5 μm, a chaotic,hierarchical structure of winding tubules is formed.

[0043]FIG. 9 is a schematic illustration of the process of thisinvention used to induce guided growth of mesoscopic silicatestructures.

[0044] FIGS. 10(a) and (b) show SEM images of 1 μm line and squaremesoscopic silicate patterns formed by guided growth withinmicrocapillaries in accordance with this invention. Electro-osmotic flowis used to transport reacting fluid through the capillaries, andlocalized Joule heating triggers rapid polymerization of the inorganicaround aligned surfactant tubules.

[0045] FIGS. 11(a) and (b) show TEM images of a patterned mesoscopicsilica structure grown on a Thermanox plastic substrate (ElectronMicroscopy Sciences) in accordance with this invention. These display ahexagonally packed surfactant tubule structure within the micron-sizedlines shown in FIG. 10. The cross-sectional view of each line reveals anidentical hexagonally packed pattern of tubules, suggesting globalalignment of tubules parallel to the substrate and capillary walls.Similar images have also been obtained for “confined” films grown onsilica substrates. The insert in this FIG. 11 displays the correspondingelectron diffiraction

DETAILED DESCRIPTION OF THE INVENTION

[0046] This invention is directed to a process of preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns. The synthesis of silica-basedmesostructured materials by using supramoleclar assemblies of surfactantmolecules to template the condensation of inorganic species is abiomimetic approach to the fabrication of organic/inorganicnanocomposites. This technique holds great promise as a synthetic schemeto produce nanostructured materials with novel properties. For any ofthese applications to be realized, however, what is required is a methodby which these nanostructures can be formed into controlled shapes andpatterns rather than the microscopic particulates that have beenpreviously reported.

[0047] The process of this invention includes forming a polycrystallineinorganic substrate having a flat surface and placing in contact withthe flat surface of the substrate a surface having a predeterminedmicroscopic pattern. A network of patterned capillaries is formed byplacing an elastomeric stamp (typically made of polydimethyl siloxane,PDMS) possessing designed relief features on its surface in contact witha flat substrate (see FIG. 9).

[0048] Preferably, an ordered silicate structure is used within a highlyconfined space, using the Micromolding in Capillaries (MIMIC) technique.Other polycrystalline inorganic substrates may also be used and includea wide variety of transition metal oxides, cadmium sulfide and selenidesemiconductors.

[0049] A preferred approach is to start with a well-defined interfacesuch as mica. Under acidic conditions, reactive SiOH anchoring sites onmica provide binding sites for the silica-surfactant micellar precursorspecies and orient a hexagonal phase of mesostructured silica as acontinuous thin film. It has been found that this approach is not justlimited to the hydrophilic surface of mica but can be generalized toform continuous mesostructured silicate films onto a wide variety ofsubstrates, including hydrophobic surfaces such as graphite. Of primaryconcern is the Structure of the first layer of adsorbed surfactant ateach of these interfaces. Although the molecular organization andself-assembly of surfactants at interfaces is a widely studied area,little is still known about the precise structure of adsorbed surfactantlayers.

[0050] An acidified aqueous reacting solution is then placed in contactwith an edge of the surface having the predetermined microscopicpattern. The solution wicks into the microscopic pattern by capillaryaction. The reacting solution has an effective amount of a silica sourceand an effective amount of a surfactant to produce a mesoscopic silicafilm upon contact of the reacting solution with the flat surface of thepolycrystalline inorganic substrate and absorption of the surfactantinto the surface.

[0051] It is important for the biomimetic processing of thin inorganicfilms to maintain relatively low levels of supersaturation during theprecipitation process in order to minimize the amount of particleformation in bulk solution.

[0052] All mineralization processes involve the precipitation ofinorganic material from solution. A key requirement for successful filmformation is to promote the formation of the inorganic phase on thesubstrate directly (that is, heterogeneous nucleation) and prevent thehomogeneous nucleation of particles in the solution. According toclassical nucleation theory, the free energy change (ΔF) associated withthe precipitation of an inorganic cluster from solution onto a surfaceis given by:

ΔF=−nk _(B) T ln S+γ _(il) A _(il)+(γ_(is)−γ_(sl))A _(is)  (1)

[0053] where S represents the degree of supersaturation in the fluid; nis the aggregation number; k_(B) is Boltzmann's constant; T istemperature; γ_(il), γ_(is), and γ_(sl) represent the inorganic/liquidinterfacial tension, respectively; and A_(il) and A_(is) represent thecorresponding interfacial areas.

[0054] When the interaction between the growing nucleus and substratesurface represents a lower net interfacial energy than theinorganic/solution interfacial energy i.e.,(γ_(is)−γ_(sl))A_(is)<γ_(il)A_(il), heterogeneous nucleation is favoredover homogeneous nucleation. This is the case for the majority ofprecipitating inorganic systems, and hence heterogeneous nucleation isthe dominant precipitation mechanism for thermodynamically controlledsystems. Homogeneous nucleation will only dominate at relatively highlevels of supersaturation where the precipitation process becomeskinetically controlled. Precipitation times for homogeneous nucleationvary enormously, from months to milliseconds, depending sensitively onthe value of S.

[0055] More specifically, in order to promote growth of a mesostructuredinorganic on these substrates, an aqueous recipe that includes an excessof adsorbing cetyltrimethyl ammonium chloride (CTAC) surfactant and adilute acidic solution of tetraethoxy silane (TEOS) inorganic precursoris used. Inorganic solute concentrations are purposefully kept dilute inorder to decrease the rate of homogeneous nucleation to such an extentthat the more thermodynamically favored heterogeneous nucleation routeis dominant. The procedure involved dissolving TEOS liquid in an aqueoussolution of CTAC and hydrochloric acid. Typical molar ratios are 1TEOS:2 CTAC:9.2 HCl:1000 H₂O.

[0056] The formation of a mesoscopic silica film begins to occurimmediately upon contact of this solution with any interface onto whichthe surfactant can adsorb. A dilute solution of the TEOS silica sourceis specifically used to prevent homogeneous nucleation of inorganicmaterial in bulk solution, and to promote heterogeneous nucleation andgrowth of a mesoscopic film at the substrate/solution interface.

[0057] Aksay et al., Science 273, 892 (1996), the entire disclosure ofwhich is incorporated herein by reference, describes the production ofmesoscopic films without the required electric field of this invention.FIGS. 1-7 are taken from this reference for background and comparison.

[0058]FIG. 2 shows SEM images of mesoscopic films grown for a period of24 hours at the mica, graphite, and silica/water interfaces. Undersimilar conditions, freestanding mesostructured silica films can also begrown at the air/water interfaces. All of the films are continuous anddisplay distinctly different textures at length scales between 0.5 and10 μm.

[0059]FIG. 3 shows in situ AFM images of the atomic lattice of eachsubstrate as well as the structure of the mesoscopic silica overlayergrowing on each surface. To obtain these images, a method was used thatuses electrical double layer repulsive forces to image the chargedistribution of an adsorbed layer on the sample. The images of the outerlayer of the reacting mesostructured film were obtained by immersing theimaging tip and substrate in the reacting mixture and, once sufficienttime was allowed for thermal and mechanical equilibration, setting theimage setpoint in the repulsive precontact region. In this way, the tipis held ˜1 nm above the reacting surface and the scanning motion of theAFM produces a topological map of charge density.

[0060] In the case of mica, FIG. 3A reveals meandering stripes with aspacing of 6.2 to 6.8 nm. These are observed at every stage of thereaction. After 10 hours of reaction, in situ AFM images are difficultto obtain because of the appreciable growth of mesostructured silica onthe top surface of the AFM flow cell and cantilever spring. The presenceand irregular nature of both of these films disturb the reflection ofthe laser light beam used to monitor spring deflection. As discussedbelow, x-ray diffraction (XRD) analysis of these films reveals adistorted hexagonal stacking of surfactant tubules (5.6 nmnearest-neighbor spacing) that lie parallel to the surface and areaxially aligned along the next-nearest-neighbor direction of thehexagonal oxygen lattice on the mica surface.

[0061]FIG. 4 shows TEM images of a mesostructured film on mica, cut intwo different transverse directions. All three methods reveal aconsistent structure of the mesostructured film on mica. AFM imagessimilar to those in FIG. 3A were obtained without TEOS present, butthese interfacial surfactant films are limited to one or two layers ofcylindrical tubules. AFM studies on systems containing only surfactant,with no TEOS, reveal the presence of several layers of adsorbedsurfactant tubules. Three-dimensional “multilayer” features have beenimaged with the microscope, and as many as three “steplike” features areobserved in the repulsive portion of the force-distance curve near thesubstrate (M. Trau et al., in preparation). The existence of suchsupramolecular surfactant structures in the absence of TEOS suggests asequential reaction mechanism involving surfactant self-assemblyfollowed by inorganic condensation.

[0062] The self-assembly of micellar layers without the presence of theinorganic agent suggests a sequential growth and polymerization for thesilicate films (FIG. 5A). First, the surfactant self-assembles on themica substrate to form meandering tubules, and second, silicon hydroxidemonomers (or multimers) polymerize at the micellar surface. Aspolymerization continues, more surfactant is adsorbed to the freshlyformed inorganic surface and allows the templated mesoscopic structureto replicate itself and grow in to the bulk solution. After growthperiods of 24 hours, the mesoscopic composite films begin to developlarger scale structural features such as those shown in FIG. 2A. At thisstage, aligned “tapes” and steps appear with macroscopic grain boundaryangles 60° and 120°. These macroscopic angles clearly result from atomiclevel registry of the surfactant tubules with the underlying micalattice.

[0063] For graphite substrates (FIGS. 2B and 3B), the surfactant tubulesare also aligned parallel to the surface, but in this case they arerigid, parallel stripes without the meandering curvature observed onmica. Measured nearest-neighbor spacings similar to that seen on micaand microscopic grain boundaries can be clearly imaged, which againsuggests a preferential axial orientation of the surfactant tubules withthe hexagonal graphite lattice. The graphite surface is distinct frommica in that it is hydrophobic and does not contain ionizable moietiesto engender surface charge. Attractive interactions (hydrophobic and vander Waals) between the graphite surface and surfactant tails cause themto adsorb horizontally (FIG. 5B), and the resulting large interactionarea per molecule gives rise to a strong orientation effect betweenmolecule and substrate that is preserved in the cylindrical aggregates.Mica interacts only with the head group and orients the adsorbedmolecules vertically; the smaller interaction area gives rise to acorrespondingly smaller orientation effect. At long reaction times,macroscopic features grow out of the oriented, uniform film similar withmacroscopic angles of 60° and 120° are also observed.

[0064] Growth of these films at the silica/water interface gives rise tosilica films with macro and microstructures dramatically different fromthe ones described above. FIG. 3C shows an in situ AFM image of thereacting film grown from a silica substrate. Rather than the parallelstripes observed on the previous substrates, this image shows periodicarrays of dimples suggesting an orientation of surfactant tubules out ofthe plane of the interface. An XRD analysis confirms a distortedhexagonal packing of the tubules. The dimpled pattern suggests atwisting arrangement of hexagonally packed tubules attached to theinterface at one end and spiraling into the solution. Similar dimpledstructures were also observed with neat CTAC solution, which suggeststhe formation of roughly spherical surfactant aggregates that act asstarting points on the surface for growth of cylindrical tubules intothe solution. Micellar structures of quaternary ammonium surfactants onsilica have been previously postulated and observed.

[0065] As in the case of mica and graphite, the structure formed in thesilica substrate films is a direct consequence of the arrangement of thefirst layer of adsorbed surfactant on the surface. It appears that theordering ability of the silica interface, which is dramaticallydifferent from that of mica and graphite, is not great enough to confinethe surfactant tubules to lie straight on planar surfaces. Indeed,having nucleated one end of the tubules at the interface, the long axesof the tubules appear to wander over a wide range of slowly curvingconfigurations in three dimensions, suggesting that it takes very littleenergy to bend the tubules along their long axes. This effect may simplybe understood in terms of a Helfrich (W. Z. Helfrich, Natur for Chung28C, 693 (1973)) bending energy model of the tubule surfactant layer:$\begin{matrix}{E = {{\frac{k_{c}}{2}\left( {\frac{1}{R_{1}} + \frac{1}{R_{2}} - \frac{1}{R_{0}}} \right)^{2}} + \frac{k_{g}}{R_{1}R_{2}}}} & (2)\end{matrix}$

[0066] where E is the free energy per unit area (effectively, an energyper surfactant molecule); k^(c) and k^(g) are the rigidity and Gaussiancurvature constants, respectively; and R¹, R₂, and R⁰ are the principleradii and the spontaneous radius of curvature, respectively. Althoughthis form was derived for the thin-film limit in which the radii arelarge compared to the thickness of the surfactant layer, it also appearsto describe reasonably well certain cases in which the surfactant layerthickness is comparable to R¹. and R₀. R₁, the small radius of thetubule, is strongly constrained by the length of the surfactantmolecules. Insofar as R⁰ is fixed by the surfactant composition and issmall (˜5 nm), as is typically the case for single-chain surfactants,and if R₂>>R¹ as is the case for long, thin tubules, then Eq. 2 is wellapproximated by: $\begin{matrix}{E = {{k_{c}/2}\left( {{1/R_{1}} - \frac{1}{R_{0}}} \right)^{2}}} & (3)\end{matrix}$

[0067] In other words, the energy of bending along the long axis of atubule does not figure prominently into the bending energy. Unless orderis imposed on the tubules by external forces, such as adsorption forces,the tubules will sample a wide range of slowly varying configurations.This prediction is also consistent with observed macroscopic structuresof mesoscopic silica films formed after long growth times at thesilica/water interface (FIG. 6). These films begin growing as veryuniform structures but soon become increasingly textured and chaotic asthe film thickness increases. Rather than the oriented tapes observed inthe cases of mica and graphite substrates, the silica substrate filmsdisplay chaotic, spiral-like structures wrapped in a hierarchicalfashion around each other (FIGS. 2C, 3C, and 6). The films becomeincreasingly disordered once the thickness is great enough such that thesurface can no longer induce ordering.

[0068] Aksay et al., investigated the substrate ordering effect onsurfactant tubules and subsequent mesoscopic silica films, by performingXRD analyses of films grown on mica. In these measurements, the growthwas terminated by partially draining the solution in a sealed cell andperforming the measurements while the sample was in contact with thevapor of the growth solution. FIG. 7 indicates that the films arestrained in the plane perpendicular to the substrate, with the hexagonalpacking of tubules distorted by as much as 4% during growth. Upondrying, strain within the film is significantly altered. It was observedby Aksay et al., that there was +18% strain in the film grown in thenonequilibrium condition, in the case where the solution is confined ina thin film geometry by a wrap. It was also found that the film growsabout an order of magnitude faster in this condition. There was a largechange in the Bragg peak position as these films dry that corresponds tolarge changes in the film lattice constant and strain. For example, thedegree of. hexagonal strain varies from +18% while wet to +2% a fewhours after removal from solution, and finally −7% several days laterwhen they are dry. In this process, the lateral spacings of the filmincreased by 2% upon drying and finally by 5% after a few days. Thisimplies that the 25% change in a/b ratio of the film upon drying waslargely due to a change in the vertical lattice spacings due to dryingshrinkage in the normal direction.

[0069] During growth, the strain appears to result from the orderinginfluence that the mica substrate exerts on the adsorbed surfactanttubules. That is, the forces that act to align the tubules parallel tothe surface also act to deform the hexagonal packing in threedimensions. The forces responsible could be either van der Waals orelectrostatic in nature because the mica surface has ionizable moieties.As the self-assembled organic layers grow away from the surface, theordering effect is expected to diminish. Experiments performed in theabsence of the TEOS inorganic precursor revealed that one or two layersof surfactant tubules can adsorb to the substrate prior to silicacondensation. Once the TEOS is included in the solution, silica beginsto condense within the adsorbed surfactant layers and films grow awayfrom the surface. More layers of surfactant can now adsorb to thefreshly formed silica interface, which provides a mechanism for the filmto continue to grow out into the solution. This growth mechanism,however, does not relieve the original strain in the film Moreover, asthe film grows thicker, tubules adsorbed to the mesostructured silicawill be stained differently to the initial layers adsorbed on the micasurface. Evidence for the eventual release of this strain is seen mostclearly on the surface of films grown for long periods. An example isseen in FIG. 2A, where macroscopic features such as the “swirling tube”and “hook” appear and grow out of the aligned film in wormlike manner.On mica, these features begin to occur at film thicknesses of ˜0.5 μmand always possess a wormlike structure. For films grown at thesilica/water interface, dramatically different structures are seen togrow out of the film at similar film thicknesses (FIGS. 2C and 6).Although the first layer of tubule structure is different for eachsubstrate, the release of accumulated strain within these films throughthe growth of tubule bundles away from the oriented film is a commonfeature of all of these films. For mica and graphite, these bundles formwormlike structures, and for silica, tapes and spirals are formed thatwrap around each other in a hierarchical manner. The hierarchicalstructures formed in thick films thus appear to result from the releaseof accumulated strain energy associated with the epitaxial mismatchbetween the first layer of adsorbed surfactant and the periodic atomiclattice of the substrate. In all cases, this is observed to occur onlyfor relatively thick films (≧0.5 μm) where the ordering influence of thesubstrate no longer exists.

[0070] The invention herein requires that after the acidified aqueousreacting solution be placed in contact with an edge of the surfacehaving the predetermined microscopic pattern, and that an electric fieldbe applied and directed tangentially to the surface within themicroscopic pattern. The electric field should be sufficient to causeelectro-osmotic fluid motion and enhance the rates of fossilization bylocalized Joule heating.

[0071] It has been found that within the capillaries, because thereacting solution is dilute, reactants are quickly depleted, and filmgrowth ceases. Moreover, the growth of mesoscopic film at the edges ofthe mold seals the capillaries and prevents diffusion of reactingspecies to the interior of the mold. To maintain a uniform concentrationof reactants within the capillaries during the growth process, theelectric field is applied parallel to the substrate in the mannerillustrated in FIG. 9.

[0072] Application of an electric field in this geometry has threeeffects: It induces elctro-osmotic fluid flow; it aligns surfactanttubules; and it causes localized Joule heating of the solution. Theseeffects are synergistic in guiding and fossilizing the silicatemesostructures within the microcapillary reaction chambers. For appliedfields >0.1 kV nm⁻¹, electro-osmotic fluid flow is observed within thecapillaries, as a result of the interaction of the field with the ionicdouble layer charge near the capillary wall. Surface charge on thecapillary walls arises from adsorption of the positively charged CTACsurfactant. Maintaining a steady fluid flow through the capillariesduring the entire growth process ensures that the reactant concentrationwithin each micro reaction chamber remains constant with time—thisconstancy allows uniform films to be grown.

[0073]FIG. 10 shows SEM images of square and lined patterns ofmesoscopic silica grown on a silica substrate after 5 hours of reactiontime. A DC field of 0.15 kV mm⁻¹ was applied during the entire reactionprocess, and fresh reacting fluid was continuously dripped on one sideof the mold to replenish the volume removed by the electro-osmotic flow.In each case, the patterns formed replicate the structures of the mold.Within the capillaries, films begin to grow on all exposed surfaces,i.e., at both the PDMS mold and the substrate/aqueous solutioninterface. As the reaction progressed, the capillaries narrowed in thecenter and eventually sealed completely.

[0074] The high conductivity of the acidic reaction solution gives riseto significant Joule heating at these applied voltages. Positioning theelectrodes in an excess reservoir of reacting solution outside themicrocapillary volume (FIG. 9) allows high fields to be applied acrossthe aqueous solution confined within the capillaries. In such a scheme,rapid electrolysis (H₂O→H₂+½O₂) ensues, however bubble formation isconfined to the fluid reservoir near each electrode and does not disturbthe formation of silica mesostructures within the capillaries. Atvoltages of 1 kV mm³¹ ¹, sparks are occasionally observed within thefluid confined in the microcapillary as a result of intense localizedheating. At lower fields, sparks are not observed and the Joule heatingaccelerates the fossilization rate of the mesoscopic silica byincreasing the rate of polymerization of TEOS precursor to silica.

[0075] With no applied field, 0.5 μm thick films are typically grown ina period of 24 h; with an applied field of 0.1 kV mm⁻¹, similarthicknesses are achieved in 1-5 h. Localized heating of the reactingsolution in this manner provides a useful method of rapidly rigidifyingthe aligned surfactant tubular structures formed within themicrocapillaries.

[0076] In order to determine the orientation of the surfactantnanotubules within these structures, cross-sectional samples wereprepared using a Leica ultramicrotome and analyzed by high resolutiontransmission electron microscopy (TEM). FIG. 11 shows a typical exampleof the resulting TEM images as well as a typical selected area electrondiffraction pattern (SAED). These reveal a hexagonally packedarrangement of tubules with a nearest neighbor spacing of 3.0 nm.Detailed examination of diffraction pattern reveals a slightly distortedpacking arrangement, with a deviation of 4% from perfect hexagonal. Thisdistortion may be a result of the accelerated fossilization processdescribed above: with no applied field, no distortion is observed.

[0077] Multiple cross-sections taken of the 1 μm line structures shownin FIG. 10 all appear identical to the image shown in FIG. 11. Thisindicates that all tubules within the capillaries are aligned parallelto the substrate, the long axis of the capillary, and the direction ofthe applied field. This results in a dramatic contrast to the“unconfined” mesoscopic silica film synthesis, which always results in achaotic and non-aligned arrangement of tubules (FIG. 8). In the processof this invention, rather than taking on a random configuration, growingtubules are guided within the confined space of the capillary and remainparallel to the walls. This orientation occurs either as a result of theaction of the external field, i.e., alignment of tubules resulting frompolarization body forces that operate in regions of dielectric constantgradient (˜∇εE²) or by virtue of the confined space within which thereaction is performed. In both cases, the tubules would be alignedparallel to the capillary walls. For field-induced alignment, suchconfigurations minimize the overall electrostatic energy—provided adifference in dielectric constant exists between the inner and outervolume of the tubule. It is also known that the formation of end-caps inself-assembled surfactant cylinders is not favored, given their highfree energy of formation. Thus, within, a highly confined region,surfactant cylinders will take on configurations which minimize thenumber of end-caps. Consequently, they will tend to elongate along thelong axis of the capillary rather than truncating at capillary walls.

[0078] In the absence of an ordering field, a wide range of slowlycurving configurations of tubules if formed in three-dimensions (FIG.8). As described previously, such a configuration can be understood interms of a simplified Helfrich, W. Z., Naturforch. 28C, 693 (1973),bending energy model of the surfactant tubule, E=K_(c)/2(1/R₁−1/R₀)₂,where E is the free energy per unit area (effectively, an energy persurfactant molecule in the tubule), k_(c) is a rigidity constant, and R₁and R₂ are respectively the principal and spontaneous radius ofcurvature of the tubule. In so far as R₀ is fixed by the surfactantcomposition and is small (˜5 nm), as is typically the case for singlechain surfactants, the above equation shows complete insensitivity to R₁for values >>R₀. This analysis implies that the energy of bending alongthe long axis of a tubule does not figure prominently in the bendingenergy. Thus, unless order is imposed on the tubules by external forces,such as adsorption forces, or an electric or flow field, the tubuleswill sample a wide range of slowly varying configurations. This concursduring “unconfined” film growth, where it was shown that orientation andalignment of tubules can be controlled in the initial stages of filmgrowth by manipulating the strength and nature of the specificsurfactant-substrate interactions. Although this growth scheme givessome control over the film structure, once the film grows away from theinterface, the orientation that existed in the first layers begins to belost as the ordering influence on the interface diminishes. In our case,the combined influence of confining geometry and applied field allowsthe synthesis of mesoscopic silicate nanostructures with preciselycontrolled geometries. In this way, the tubule geometry is controlled inall regions of the film and the synthesis can be performed on anyrequired substrate, regardless of the nature of the surfactant-substrateinteraction.

[0079] An enormous variety of patterns can be formed using the MIMICapproach, with nanotubules aligned parallel to capillary walls.Capillary thicknesses of 1 μm, corresponding to roughly 300 nanotubules,are easily achieved by this method and thinner structures can also beformed using molds formed from masters prepared by electron beamlithography.

[0080] As a viable method for the production of thin films with complexnanometer and micro-scaled hierarchical architecture, the guided growthof mesoscopic silicates within confined geometries provides a convenientmethod for fabrication of nanostructured materials in a variety ofapplications ranging from sensors and actuators to optoelectronicdevices.

[0081] Having thus described the invention in detail, it is to beunderstood that the foregoing description is not intended to limit thespirit and scope thereof What is desired to be protected by LettersPatent is set forth in the appended claims.

What is claimed is:
 1. A process for preparingsurfactant-polycrystalline inorganic nanostructured materials havingdesigned microscopic patterns, comprising: a) forming a polycrystallineinorganic substrate having a flat surface; b) placing in contact withthe flat surface of the substrate, a surface having a predeterminedmicroscopic pattern; c) placing in contact with an edge of the surfacehaving the predetermined microscopic pattern, an acidified aqueousreacting solution, the solution wicking into the microscopic pattern bycapillary action, wherein the reacting solution comprises an effectiveamount of a silica source and an effective amount of a surfactant toproduce a mesoscopic silica film upon contact of the reacting solutionwith the flat surface of the polycrystalline inorganic substrate andabsorption of the surfactant into the surface; and d) applying anelectric field tangentially directed to the surface within themicroscopic pattern, the electric field being sufficient to causeelectro-osmotic fluid motion and enhanced rates of fossilization bylocalized Joule heating.
 2. A process for preparing surfactant-siticatenanostructured materials having designed microscopic patterns,comprising: a) forming an ordered silicate structure substrate having aflat surface; b) placing in contact with the flat surface of thesubstrate a surface of a stamp, the stamp surface having relief featurescomprising a predetermined microscopic pattern; c) placing in contactwith an edge of the stamp an acidified aqueous reacting solution, thesolution wicking into the predetermined microscopic pattern by capillaryaction, wherein the reacting solution comprises an effective amount of asilica source and an effective amount of a surfactant to produce amesoscopic silica film upon contact of the reacting solution with theflat surface of the ordered silicate structure substrate and absorptionof the surfactant into the surface; and d) applying an electric fieldtangentially directed to the surface within the microscopic pattern, theelectric field being sufficient to cause electro-osmotic fluid motionand enhanced rates of fossilization by localized Joule heating.
 3. Aprocess for preparing surfactant-silicate nanotubule structures havingoriented patterning, comprising: a) forming an ordered silicatestructure substrate having a flat surface; b) placing in contact withthe flat surface of the substrate a surface of an elastomeric stamp, theelastomeric stamp surface having design relief features comprising anetwork of channels; c) placing in contact with an edge of the stamp anacidified aqueous reacting solution, the solution wicking into thenetwork of channels by capillary action, wherein the reacting solutioncomprises an effective amount of a silica source and an effective amountof a surfactant to produce a mesoscopic silica film upon contact of thereacting solution with the flat surface of the ordered silicatestructure substrate and absorption of the surfactant into the surface;and d) applying an electric field tangentially directed to the surfacewithin the channels, the electric field being sufficient to causeelectro-osmotic fluid motion and enhanced rates of fossilization bylocalized Joule heating.
 4. A process for preparing surfactant-silicatenanotubule structures having oriented patterning, comprising: a) formingan ordered silicate structure substrate having a flat surface; b)placing in contact with the flat surface of the substrate a surface ofan elastomeric stamp, the elastomeric stamp surface having design relieffeatures comprising a network of channels; c) placing in contact with anedge of the stamp an acidified aqueous reacting solution the solutionwicking into the network of channels by capillary action, wherein thereacting solution comprises an effective amount of tetraethoxysilane(TEOS) and an effective amount of a surfactant to produce a mesoscopicsilica film upon contact of the reacting solution with the flat surfaceof the ordered silicate structure substrate and absorption of thesurfactant into the surface; and d) applying an electric fieldtangentially directed to the surface within the network of channels, theelectric field being sufficient to cause electro-osmotic fluid motionand enhanced rates of fossilization by localized Joule heating.
 5. Theprocess of claim 2, 3 or 4, further comprising removing the stamp afterfossilization.
 6. The process of claim 4, wherein the effective amountof tetraethoxysilane (TEOS) and the effective amount of the surfactantare sufficiently dilute to prevent homogeneous nucleation oftetraethoxysilane (TEOS) inorganic material in the solution prior toplacing the solution in contact with the edge of the stamp.
 7. Theprocess of claim 2, 3 or 4, wherein the surfactant iscetyltrimethylammonium chloride (CTAC).
 8. The process of claim 2, 3 or4, wherein the ordered silicate structure is produced within a highlyconfined space using the Micromolding in Capillaries (MIMIC) technique.9. The process of claim 3 or 4, wherein the elastomeric stamp comprisespolydimethyl siloxane, (PDMS).
 10. The process of claim 7, wherein thereacting solution has a molar ratio of about 1 TEOS :1.2 CTAC: 9.2 HCI:1000 H₂P.
 11. The process of claim 1, 2, 3 or 4, wherein the electricfield is about 0.1-1 kVmm⁻¹.